Aluminum base bearing



ALUMINUM EASE BEARING Alfred W. Schluchter, Dearborn, Mich, assignor to General Motors Corporation, Detroit, Micln, a corporation of Delaware No Drawing. Application October 6, 195'1, Serial No. 250,191

Claims. (Cl. 75-147) This invention relates to an aluminum base alloy and particularly to an improved alloy of this type having properties rendering it especially suitable for use as a bearing material.

Many aluminum base bearing alloys, such as the type disclosed in Patent No. 2,238,399, which issued April 15, 1941 in the name of Alfred W. Schluchter, are satisfactory bearing materials in most respects. However, such alloys cannot be satisfactorily heat treated so as to provide suificient hardness for many purposes. Accordingly, a principal object of my invention is to provide an aluminum base bearing alloy which can be heat treated so that it possesses a hardness comparable to that of any conventional hardenable aluminum alloy and which, at the same time, can be rolled into strip form by conventional commercial methods.

A further object of this invention is to provide such a heat treatable aluminum alloy which has exceptionally good score resistance when used as a bearing. Aluminum and most of its alloys are generally quite unsuitable for use in bearings for machine parts of iron for the additional reason that aluminum tends to adhere to, or combine with, the ferrous metal, thereby causing scoring or seizing. I have found, however, that by a suitable combination of alloying constituents this difficulty can be overcome and an alloy produced having not only excellent anti-friction properties but other characteristics especially desirable in a bearing material.

In accordance with my invention, therefore, the foregoing and other objects and advantages are attained to a particularly high degree in an aluminum base alloy containing chromium, magnesium, cadmium and silicon. Inasmuch as the alloy thus produced is a much stronger metal than the aluminum alloys heretofore used for bearing purposes, solid bearings may be made from it, no backing of steel or similar metals being necessary for many applications. Of course, this alloy can also be readily bonded to steel and many other metals and can be used on a backing.

Furthermore, the above-described alloy is characterized by much greater hardness than related aluminum base alloys heretofore used, heat treatment of this alloy resulting in increasing its hardness as much as several hundred percent. Such a high degree of hardness is desirable because recently developed high compression engines impose exceptionally heavy loads on bearings, thus creating an increased need in recent years for greater hardness in such bearings. Similarly, the greater hardness of my alloy permits it to be formed into a bearing having a correspondingly longer fatigue life. As a result of this hard ness, solid bearings made from this alloy also retain their original shapes much better than many of the bearings which heretofore have been made of softer alloys. The former do not take a set at temperatures to which they are normally subjected, and they undergo a negligible amount of shrinkage when removed from engines after 'tensive use. Despite these aforementioned properties,

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2,766,1i5 Patented Oct. 9, 1956 the alloy can be easily rolled down by conventional methods.

In accordance with my invention, highly satisfactory bearing properties are obtained with an alloy having the following composition by weight: 0.05% to 3.0% magnesium, 0.05% to 5.0% cadmium, 0.3% to 11.0% silicon, 0.1% to 2.25% chromium and the balance substantially all aluminum. Various incidental impurities may be included in this alloy in the usual small amounts without any substantial detrimental effects. For example, iron, which together with silicon is present in commercial aluminum, may be present in amounts up to 0.5% Without causing any harmful results. Under severe test conditions, alloys having the above composition show excellent anti-friction properties so that bearings formed of this alloy not only do not score or gall when in contact with a rotating steel shaft, but neither the shaft nor the bearings show an appreciable amount of wear after long and severe use. Similarly, resistance to cracking or crumbling is also extraordinary.

The magnesium is added to increase the hardness of the bearing alloy, a magnesium content of only 0.05% being sufficient to provide a sufficient degree of hardness for many applications. Inasmuch as the molten magnesium tends to oxidize during the alloying procedure, however, for best results it is preferable that the magnesium be added in amounts equal to at least 0.2% of the weight of the alloy. Magnesium has an adverse effect on score resistance and friction properties, however, and as a result the magnesium content should not be higher than approximately 3.0%.

With additions of magnesium in amounts greater than approximately 0.5%, the increase in hardness is relatively slight. Moreover, if the magnesium content is not higher than this amount, the addition of chromium tends to offset the adverse effect of magnesium on the score properties of the alloy. Accordingly, a magnesium content ranging from 0.2% to 0.5% is preferred, approximately 0.5% magnesium generally being the optimum amount to be added.

The addition of cadmium greatly improves the score resistance of the alloy. Despite the fact that it has been generally recognized that the addition of cadmium to aluminum causes slight loss of strength, I have found that cadmium, in the presence of silicon, may be beneficially introduced in amounts as large as 5.0% without causing loss of strength. In fact, the resultant alloy is remarkably resistant to disintegration under impact or pounding such as occurs in severe bearing service. Moreover, the presence of cadmium does not effect the hardness if the alloy is subsequently heat treated. Although the effect of cadmium on both strength and hardness is negligible in any event if added in quantities no greater than 5.0%, cadmium is a relatively soft metal and hence the cadmium content should not be higher than this amount.

I have also found that a cadmiumcontent greater than 5.0% tends to cause this element to segregate out and settle to the bottom of the casting during the solidification thereof in the form of the apparently nearly pure metal. Thus, a too high cadmium content raises the cost of the alloy by increasing personnel expenses beamount ranging from approximately 0.2% to 2.5% in order to provide the most desirable anti-friction properties. Inasmuch as cadmium also tends to volatilize at the temperature of molten aluminum, however, it often may be desirable to add slightly greater amounts of cadmium to offset this tendency for volatilization. A cadmium content of at least 0.05% is necessary in all instances to provide adequate score resistance.

The inclusion of silicon in my aluminum base bearing alloy also enhances its score resistance. This property of silicon, plus the manner in which it influences the effects of the cadmium present in the allow and the fact that soldification shrinkage is lower as the silicon content is raised, dictates that the alloy contain at least 0.3% silicon. inasmuch as a high silicon content interferes with rolling processes, however, the maximum amount of silicon to be added necessarily is governed by the method in which the article, such as a bearing, is formed. Accordingly, silicon should not be present in amounts greater than 5.0% in the wrought alloy because such an alloy needs to be rolled, While it may be added in amounts as high as 11.0% in the cast alloy. While an increased silicon content improves score resistance, the addition of silicon in amounts greater than 5.0% provides only slight aditional beneficial properties in this respect. Accordingly, best results are obtained for most purposes when the silicon content is kept within a preferred range of 2.0% to 5.0%.

The adition of chromium, in conjunction with the magnesium present in the aluminum base alloy, contributes to the hardenability of the resultant alloy. Moreover, chromium is also particularly beneficial in that it improves the score properties of the alloy by compensating for the detrimental effects of magnesium on score resistance. These desirable qualities relative to hardness and score resistance are provided by adding chromium in amounts ranging from approximately 0.1% to 2.25

While the hardness of the alloy will be substantially reduced if the chromium content is too low, the addition of only approximately 0.5% chromium is all that is necessary in order to obtain a completely satisfactory degree of hardness. On the other hand, the score resistance of the alloy is slightly improved as the chromium content is increased. Furthermore, the addition of chromium in amounts greater than 2.25% reduces the ductility of the resultant alloy to too great an extent, a high ductility being necessary in wrought alloys. Also it is not feasible to add more than 2.25% chromium because increasing the chromium content above this amount raises alloy costs by greatly increasing the difficulty in casting and fabrication of the cast parts. Too high a temperature is required to place and hold greater quantities of chromium in solution in the liquid state, the chromium segregating out unless the temperature of the melt is raised excessively. Inasmuch as cadmium volatilizes excessively above approximately 1400" F., a 2.25% chromium content is therefore about the upper limit that can be used with conventional foundry equipment, this being the saturation point of the chromium in the aluminum alloy at this temperature.

As a result of the above considerations, I have found that a chromium content within a preferred range of 0.25% to 0.75% provides excellent results in all respects.

In the alloy hereinbefore described, it is necessary that both magnesium and chromium be used in conjunction to obtain the desired hardness. The use of either one of these metals alone in a quantity equal to the combined amounts of the two metals will not provide the same degree of hardness as the use of the two metals in combination.

The above alloy possesses the aforementioned desirable characteristics to an outstanding degree when it consists of the following preferred composition: 0.5% magnesium, 2.0% cadmium, 4.0% silicon, 0.5% chromium and the balance substantially all aluminum. As hereinbefore stated, other incidental impurities may be present in the Cir above alloy, but for best results the amounts of these other elements should be confined to relatively low proportions. Accordingly, it is desirable that iron, for example, be present in amounts not greater than 0.5%.

In order to obtain the high degree of resistance to pounding, such as is encountered in a hearing, it is preferable that the alloy have a physical structure typified by the absence of continuous networks of metallic elements. Conventional alloying procedures may be employed with intermediate alloys, such as aluminum-silicon and aluminum-chromium alloys, being used to introduce the silicon and chromium. It is desirable that the more volatile elements, such as the magnesium and cadmium, be the last to be added to the melt in order to prevent their vaporization. In general, it is advisable to use the lowest temperature possible to keep the cadmium from vaporizing. For example, I have found that the aluminum, silicon and chromium may advantageously be fused at a temperature in the order of approximately 1200 F., the melt then preferably being removed from the furnace. The magnesium and cadmium may next be successively added to the melt, which is subsequently stirred and cast, usually in metal or graphite molds. The highest temperature suitable for casting is that point at which the cadmium just begins to vaporize or smoke and, in order to avoid loss of metal, it is desirable not to raise the temperature of the melt above this point. Accordingly, care should be taken to prevent the temperature from exceeding approximately 1400 F. The alloy may be either cast in the desired form for use in bearings or it may be cast in ingots, rolled down to strip material of the desired thickness, and bearing liners or other hearing elements formed from the stock.

Cast articles having a metallographic structure showing a continuous network of segregated metal compounds may be improved as to strength and fatigue resistance by suitable heat treatment. For example, I have found that a solution treatment at a temperature between approximately 900 F. and 950 F. for a period of twelve to fifteen hours is particularly effective to more completely dissolve the constituent elements and form a solid solution. Upon removing the alloy from the furnace following the solution treatment, it is preferable to cool it immediately by quenching in water. This treatment provides the alloy with the high degree of ductility, such as is desirable for rolling operations; and it may then be easily rolled down to strip material of the desired thickness.

A precipitation treatment may thereafter be employed to substantially increase the hardness of the alloy. This process is preferably carried out by heating the article for five to ten hours at a temperature in the range between approximately 350 F. and 400 F., a precipitation treatment at 370 F. for eight hours being particularly satisfactory. The alloy then may be again cooled, preferably in water, and suitably machined. Such a heat treating process results in an article which is thre or four times as hard as it was in the as-cast condition and whose fatigue strength is proportionally improved.

The specific gravity of the above described alloy is about one-third that of a tin-bronze bearing alloy, and has much greater resistance to fatigue or to cracking under the pounding action to which bearings, such as connecting rod bearings, are subjected. This property renders such an alloy particularly suitable as a bearing for use under extreme conditions, tests on such bearings indicating the remarkable absence of wear, either of the hearing or the shaft. In addition, the alloy appears to be resistant to corrosion by acid constituents of lubricating oils which attack many other bearing compositions.

It is to be understood that, while the invention has been described in conjunction with certain specifiic examples, the scope of the invention is not to be limited thereby except as defined in the appended claims.

I claim:

1. A bearing formed of an alloy consisting essentially of from 0.05% to 3% magnesium, 0.05% to 5% cadmium, 0.3% to 11% silicon, 0.1% to 2.25% chromium, to 0.5% iron and the balance aluminum.

2. A bearing characterized by high anti-friction properties and resistance to disintegration under impact and to attack by acids developed in lubricating oils, said bearing being formed of an alloy consisting of from 0.05% to 3% magnesium, 0.05% to cadmium, 0.3% to 11% silicon, 0.1% to 2.25% chromium, and the balance aluminum plus incidental impurities 3. A bearing formed of an alloy capable of being rolled into sheet form from cast ingots and having high antifriction properties and fatigue resistance, said alloy consisting of 0.2 to 0.5% magnesium, 0.2% to 2.5% cadmium, 2% to 5% silicon, 0.25 to 0.75% chromium and the balance aluminum plus incidental impurities.

4. A heat-treated and Worked, corrosion resistant bearing consisting essentially of from 0.05 to 3% magnesium, 0.05% to 5% cadmium, 0.3% to 5% silicon, 0.10% to 0.75% chromium, iron not in excess of 0.5 and the balance aluminum.

5. A bearing formed of an alloy consisting essentially of 0.2 to 0.5 magnesium, 0.2% to 2.5% cadmium, 2% to 5% silicon, 0.25% to 0.75% chromium, 0% to 0.5% iron, and the balance aluminum.

References Cited in the file of this patent UNITED STATES PATENTS 1,899,133 Bosshard Feb. 28, 1933 1,945,297 Sterner-Rainer Jan. 30, 1934 1,974,971 Pacz Sept. 25, 1934 2,238,399 Schluchter Apr. 15, 1941 2,286,627 Kempf et al June 16, 1942 2,333,227 Bagley Nov. 2, 1943 2,418,881 Hensel et a1. Apr. 15, 1947 2,531,910 Hensel et a1 Nov. 23, 1950 FOREIGN PATENTS 367,831 Great Britain Feb. 22, 1932 406,638 Great Britain June 2, 1932 OTHER REFERENCES Garre: APC Serial No. 327,066, May 4. 1943. 

1. A BEARING FORMED OF AN ALLOY CONSISTING ESSENTIALLY OF FROM 0.05% TO 3% MAGNESIUM, 0.05% TO 5% CADMIUM, 0.3% TO 11% SILICON, 0.1% TO 2.25% CHROMIUM, 0% TO 0.5% IRON AND THE BALANCE ALUMINUM. 